Boston bomb suspect seriously ill

Armed guards protected the hospital where the wounded surviving Boston Marathon bombing suspect was in serious condition on Saturday and unable to be questioned to determine the motives behind the worst terrorist attack in the US since September 11, 2001.

US officials said a special interrogation team for high-value suspects was waiting to question 19-year-old Dzhokhar Tsarnaev, whose older brother and alleged accomplice was killed on Friday morning in a shootout in suburban Boston.

Authorities planned to invoke a rare public safety exception to enable the team to interrogate Tsarnaev without first advising him of his right to remain silent to avoid self-incrimination and be provided an attorney, a warning typically given to criminal suspects .

The FBI's website says the exception "permits law enforcement to engage in a limited and focused unwarned interrogation" of a suspect and introduce any statements gathered as evidence in a criminal prosecution. The FBI says "police officers confronting situations that create a danger to themselves or others may ask questions designed to neutraliae the threat without first providing a warning of rights."

Massachusetts Gov Deval Patrick, speaking outside Fenway Park after appearing at a pre-game ceremony at Saturday afternoon's Boston Red Sox baseball game, said Tsarnaev is likely "not able to communicate yet". The capture of Dzhokhar Tsarnaev lifted days of anxiety for Boston and Americans everywhere, but little was known about the motivation of the ethnic Chechen brothers.

President Barack Obama vowed investigators would solve that mystery. "The families of those killed so senselessly deserve answers," said Obama, who branded the suspects "terrorists."

Late on Friday, less than an hour after authorities said the search for the 19-year-old college student had proved fruitless and lifted a day-long order that had kept Boston-area residents in their homes, a man emerged from his Watertown home and noticed blood on the pleasure boat parked in his backyard. He lifted the tarp and found the wounded Tsarnaev, known the world over as Suspect No. 2.

Soon after that, the 24-hour drama that had shut down a metropolitan area of millions while legions of police went house to house looking for the remaining suspected Boston Marathon bomber was over. Boston police announced via Twitter that Tsarnaev was in custody. They later wrote: "CAPTURED!!! The hunt is over. The search is done. The terror is over. And justice has won. Suspect in custody."

Dzhokhar Tsarnaev's capture touched off raucous celebrations in and around Boston, with chants of "USA, USA" as residents flooded the streets in relief and jubilation after four tense days since twin explosions ripped through the marathon's crowd at the finish line on Monday, killing three people and wounding more than 180.

During a long night of violence on Thursday and into Friday, the brothers are said to have carjacked a man in a Mercedes-Benz in Cambridge, just across the Charles River from Boston, then released him unharmed at a gas station, authorities said. They are also alleged to have shot to death a Massachusetts Institute of Technology police officer, 26-year-old Sean Collier, while he was responding to a report of a disturbance, investigators said.